In your Main CSS you need to find where that background image is called. Look for it in the body attributes > background. You should find something like this:
Code:
background: #57D5FE url(images/styles/desert_green/gradients/yoshi.jpg);
To get it not to tile you need to replace the repeat with a no-repeat, for example:
HTML Code:
background: #57D5FE url(images/styles/desert_green/gradients/yoshi.jpg);
no-repeat
I think that should work.